Criterion D, E and F for 2011 - 2012:



Criteria A One World - enables students to gain better understanding of the role of society.

Criteria B Communication in science - enables students to be competent and confident when communicating information in science.

Criteria C Knowledge and Understanding in science - enables students to understand scientific knowledge and apply it to construct scientific explanations, solve problems and formulate scientifically supported arguments.

Criteria D Scientific Inquiry - enables students to develop intellectual and practical skills to design and carry out scientific investigations independently and to evaluate the experimental design (method)

Criteria E Processing Data - enables students to collect, process and interpret sufficient qualitative and/or quantitative data to draw appropriate conclusions. Students are expected to develop analytical thinking skills to interpret data and judge the reliability of the data

Criteria F Attitudes in Science - encourages students to develop safe, responsible and collaborative working practices in practical science

If you explain, the writing tends to be about facts and reasons, so you'd write about why or how something happened.
If you describe, it's like taking a photo of something - you need information, but also lots of interesting details. You can include your own thoughts and feelings as well, because that helps convey information to your audience.